The present invention has arisen out of experimental studies investigating a newly-identified biologically active substance having a high lipolytic activity which seems to be specifically associated with a range of malignant rumours, especially cachexia-inducing tumours, in animals and in humans, as disclosed for example in the specification of our European patent application No. 89302740.9 (EU0335550) of which the content is incorporated herein by reference. During the course of these studies, EPA was found to be an effective antagonist or inhibitor of this so-called lipolytic factor; it was also found to be effective in depressing an abnormal elevated level of cyclic adenylic acid (cAMP) produced in adipose tissue cells (adipocytes) by this so-called lipolytic factor or by other known lipolytically active substances, as evidenced by experiments conducted using mouse adipose tissue cell preparations. These are newly-identified properties of EPA not hitherto reported before the priority date of the present application.